Objective To assess the epidemic trend of COVID-19 Omicron and the effectiveness of containment measures in Shanghai by estimating the time-varying reproduction number （R_t ）.

Methods Based on the daily reported confirmed cases and asymptomatic infections in Shanghai from February 20 to April 26， 2022， the R package "Epiestim"， which was built by Bayesian framework method， was used to estimate the variation curve of R_t during the epidemic period and to analyze the trend of the epidemic.

Results In the early stage of the epidemic， after the implementation of school closure and nuclear acid screening in some communities， R_t continued to fluctuate between 2.000 and 3.000， reaching a peak of 2.740 （95%CI： 2.640‒2.830） on March 21， but began to decline around one week after the city lock-down on April 1. As of April 18， the R_t value in Shanghai was below the threshold of 1.000 for the first time， reaching 0.955 （95%CI： 0.951‒0.961）.

Conclusion After the implementation of public health measures with increasing strength of containment in Shanghai， the transmission rate gradually decreased， reflecting the effectiveness of the interventions. In the actual prevention and control process， the containment work should not be relaxed in order to keep the R_t below 1.000.